This invention relates in general to mechanical positioning systems. More specifically, it relates to a precision positioning system for a web coater which controls the coating profile across a one to fifteen foot web to within a tenth of a thousandth of an inch despite variation in the web thickness or other operating parameters.
For many applications it is necessary to apply a coating material to a wide, traveling web. A typical application is coating a thin layer of a hot melt adhesive onto a web of a paper, plastic or metal foil having a width of one to fifteen feet and a thickness of 4 mils, and with a variation in that thickness of .+-.5%. Ideally the coating thickness should be controlled to within tenths of a mil despite variations in the web thickness, its width, or the type of material being coated. Known coaters are not able to meet these requirements.
A first difficulty has been the die itself. It is very difficult to create a die which reliably and repeatably produces a coating of uniform thickness across the die, particularly when there are changes in the material being coated or the flow rate of coating material through the die.
The Park coater, described in U.S. Pat. No. 3,854,441, is an improvement over prior slot dies. It utilizes a fixed die and a movable backing roll that carries the web. This coater extrudes all of the coating material fed into it using a variable rate pump external to the die. There is no positive control of the coating thickness; the only control is the indirect one provided by an adjustment of the flow rate via an adjustment in the pump speed. Thus while Park coater is a significant improvement over earlier coating apparatus, it has not provided the aforementioned precision control over the coating thickness, nor has it proven to be useful for a wide range of coating materials. It has been useful with hot-melt materials, but less than optimal with water and solvent based materials.
A conventional extrusion die for coating is the "coat hanger" die which uses a contoured cavity precision ground into the die to distribute the material across the web. This type of die is costly to manufacture, is sensitive to the material being coated and the coating flow rate, and usually produces a heavier coating near the center of the web. More recently the assignee of the present application introduced a gear-in-die ("GID") coater which has a set of driven gears acting as a pump located within the die to meter the flow. This die has proven to be substantially insensitive to the material being coated and therefore it is an improvement over the Park coater and extrusion-type dies. However, the present positioning system for this GID coater has several significant drawbacks.
The GID coater has a pair of massive pivoting arms which support the die between them at an upper end and are pivotally mounted to a frame at the lower end. A torsion bar, and the die itself, couple the arms. The arms mount a motor and a chain drive for the gear pump in the die. This mounting system thus positions a die weighing approximately 1,000 to 1,200 pounds on the ends of two pivoted arms. Two large, main pneumatic cylinders are coupled to the arms to pivot the die toward and away from the web carried around a fixed backing roll.
This positioning system is characterized by less than optimal rigidity in the arms and a large angular momentum which resists rapid movement required to "jump" a splice, and in general has proven to be difficult to position and move with the desired precision. In particular, the force of an operator leaning on the die to make adjustments on the die has been sufficient to distort the die position and result in non-uniform coatings.
Various mechanical positioning systems are known from the machine tool arts, but the conventional slide and gib arrangement typically produces a positioning accurate to within only a mil. Using preloaded ball bearing and sophisticated lubrication systems, machine tools can overcome static friction and lost motion problems to approach the degree of positioning control envisioned for the present coater applications, but only with difficulty. Also, in machine tool design a movable platform is typically slide mounted on a pair of ways with the platform rigidly coupled between them. In other words, it is undesirable to have parallel slide paths where independent motion is possible along each path. In the coater art, this rigidity interferes with an accommodation to cross web thicknesses so that the coating is of uniform thickness even if the underlying web is not.
One special order coater has attempted to position a die with respect to a backing roll with the die mounted for linear, horizontal movement. The mount, however, involved a cumbersome mechanical linkage to move the die. The friction and lost motion problems allowed only a coarse positioning.
It is therefore a principal object of the present invention to provide a precision positioning system for a coater that controls the cross-web orientation of the die and its spacing from a backing roll, and a web carried on the backing roll, to within a tenth of a mil.
Another principal object of this invention is to provide a positioning system with the foregoing advantage which reliably positions and re-positions the die to a precisely known and adjustable position.
A further object of this invention is to provide a positioning system with the foregoing advantages that can also jump splices in the web.
Another significant advantage of the invention is to provide a system that is readily adapted to full servo control to provide fully automatic and continuous control of the cross-web coating profile.
Still another object is to provide a positioning system that applies a coating of closely-controlled, uniform thickness despite cross-web variations in the thickness of the web.
Another object is to provide a positioning system for a gear-in-die extrusion die where the gears are driven directly.
A still further object is to provide a positioning system with all of the foregoing advantages which has a favorable cost of manufacture as compared to comparable systems for GID coaters.